The Department of Justice filed a brief today in the case of a controversial California ballot initiative that defines marriage as between one man and one woman, asking the Supreme Court to affirm a lower court decision that struck down the measure, known as Proposition 8.

The brief marked the first time that the Obama administration has come out in court against Prop 8 and the first time it has argued against state gay marriage ban before the Supreme Court.

The administration's argument also could reverberate beyond California. If the Supreme Court accepts the arguments in the brief, it could lay the groundwork toward undermining laws against gay marriage in several other states.

"The exclusion of gay and lesbian couples from marriage does not substantially further any important governmental interest. Proposition 8 thus violates equal protection," wrote Solicitor General Donald B. Verrilli Jr. in a "friend of the court" brief filed in favor of gay and lesbian couples challenging Prop 8.

Now that it has filed a Supreme Court amicus brief, the Obama administration most likely will be granted time to actually argue its position in front of the justices. When the government files an amicus brief in an important case, it is normally granted time to argue.

A statement on behalf of ProtectMarriage.com, the proponent of Prop 8, described the Obama administration's decision to weigh in both "unprecedented" and "hardly surprising, but nevertheless disturbing."

"In his first term as president, Obama clearly stated that Americans can choose a special designation of marriage between man and woman, and that supporters of traditional marriage can hold that position without animus," said the statement by Andy Pugno, general counsel for ProtectMarriage.com. "He later remarked that it would be a 'mistake' to make the debate over redefining marriage into a federal issue.

"Yet today," the statement added, "by stating that the traditional definition of marriage is rooted only in irrational prejudice, the president has impugned the motives and actions of millions of Californians and turned his back on society's long-standing interest in both mothers and fathers raising the next generation."

Thomas Peters, communications director for the National Organization for Marriage, another gay marriage opponent, invoked California's voters in saying the group expected the court to uphold the law.

"NOM expects the Supreme Court to exonerate the votes of over 7 million Californians to protect marriage," said Peters. "The president is clearly fulfilling a campaign promise to wealthy gay marriage donors. There is no right to redefine marriage in our Constitution."

However, referring to proponents of the voter-approved measure who are defending it in court, Verrilli wrote, "Petitioners contend that Proposition 8 serves an interest in returning the issue of marriage to the democratic process, but use of a voter initiative to promote democratic self-governance cannot save a law like Proposition 8 that would otherwise violate equal protection."

While the brief fell short of calling for a fundamental right to marriage under the Constitution, it suggested that if the court were to agree with the administration's position, gay marriage laws in seven other states could be in jeopardy. Those states are Delaware, Hawaii, Illinois, Nevada, New Jersey, Oregon and Rhode Island.

"The brief pays closest attention to California and the other seven states that grant same-sex couples all the rights and responsibilities of marriage but insist on denying them the favored name," said Jane S. Schacter, a professor at Stanford Law School. "But it advocates that the court adopt a much tougher, more skeptical approach to any state law that denies same-sex couples the right to marry.

"That approach is what lawyers call 'heightened scrutiny,'" she added, "and if it were faithfully applied to all state laws banning same-sex marriage, it would result in the invalidation of those laws. The administration's brief provides a blueprint for a national right-to-marriage equality, even though it does not advocate that in express terms."

Stephen I. Vladeck, a professor of law at American University of Law, called it the "tip of a much larger anti-discrimination iceberg."

According the brief, Vladeck said, "states can't discriminate against gays without a really strong reason -- not just with respect to marriage, but adoption, employment, benefits and so on."

Today, 39 states have laws restricting marriage to opposite-sex couples. This number includes voter-approved constitutional amendments in 30 states barring same sex marriage. Nine states allow gay marriage.

"The brief filed by the solicitor general is a powerful statement that Proposition 8 cannot be squared with the principles of equality upon which this nation was founded," said Adam Umhoefer executive director of the American Foundation for Equal Rights (AFER), the group behind the challenge of Prop 8.